1. Field of the Invention
The invention relates to the production of carbon articles used in high temperature applications and, more specifically, to a technique for treating carbon articles to minimize corrosion and wear upon contact with high temperature fluids.
2. Description of the Prior Art
Carbons and graphites have been used in elevated temperature applications due to the exceptional properties that these materials exhibit at high temperature. For convenience, these carbonaceous materials hereafter will be referred to collectively as "graphite." The structural strength of articles made from graphite can be maintained through a broad range of elevated temperatures. The terms "high temperature" and "elevated temperature" as used herein refer to temperatures generally within a range of approximately 500.degree.-1000.degree. C. The strength of articles made from graphite actually increases as temperature is elevated. Articles made from graphite have been used in many high temperature applications such as electrodes (both for gouging operations and for electric arc furnaces), furnace liners, and other parts which require refractory characteristics such as crucibles for molten metal, molten metal filters, and molten metal pumps. More recently, graphite articles have been used in a broad range of jet engine and aeronautical applications such as high temperature seals.
A problem with the use of graphite in elevated temperature applications is that graphite, when in contact with air, is susceptible to corrosion as a result of high temperature oxidation. The terms "corrosion" and "oxidation" will be used interchangeably hereinafter to indicate a chemical change in the graphite wherein the carbon atoms are combined with oxygen. Corrosion causes graphite articles to deteriorate rapidly, thus requiring relatively frequent replacement with the attendant high cost thereof. In addition to corrosion problems, graphite articles brought into contact with moving molten metals or other high temperature fluids are subject to wear caused by the movement of the fluids over the surface of the articles. The abrasive action of the high temperature fluids likewise causes relatively frequent replacement of the articles which is, of course, also associated with high costs.
Many attempts have been made to diminish or correct the problems of corrosion and wear of graphite articles in high temperature applications. One known approach to the oxidation and erosion problems has employed a thin coating of silicon carbide (SiC) or silicon oxicarbide (SiOC) formed on the surface of the graphite articles. Silicon carbide and silicon oxicarbide coatings are noted for their high abrasion-resistant characteristics.
A chemical vapor deposition technique for silicon carbide is described in E. L. Kern, et al, Fabricating SiC Parts By Chemical Vapor Deposition, Solid State Research, Dow Corning Corporation, Hemlock, Mich. (approximately 1968), the disclosure of which is incorporated herein by reference. In the referenced chemical vapor deposition technique of Dow Corning, after heating the graphite article, silicon carbide is deposited on the surface of the article by contact of gaseous chemicals such as alkylchlorosilanes and hydrogen. The combined gases decompose on the surface of the article and react with the surface layer of carbon atoms to provide a high quality silicon carbide layer integrally formed with the surface of the article itself. Temperatures on the order of 1150.degree.-1250.degree. C. have been found suitable for forming layers of silicon carbide.
Unfortunately, the experience with graphite articles coated in the foregoing fashion has been that the coating tends to flake off the base material, thus exposing the base material to oxidation. Furthermore, the process of coating by itself changes the shape of the article, requiring expensive grinding of the very hard coating to maintain tolerance. Furthermore, the chemical vapor deposition technique described by Kern requires the use of isotropic graphite in molded shape, which is several times as expensive as extruded graphite.
In order to avoid cracking of the coating when subjected to thermal shock, it has been suggested that the correct approach is to first coat the graphite article with a thin layer of either silicon carbide or silicon oxicarbide followed by coating the article with a glaze. The parameters for the glaze have been that (a) it should have a coefficient of thermal expansion as close to that of the graphite article as possible, (b) that none of the constituents of the glaze should react with carbon, silicon carbide, or silicon oxicarbide in the temperature range for which the article is to be used, and (c) that the glaze should adhere well to the coating of either silicon carbide or silicon oxicarbide. Glazes that have been suggested are combinations of fluorides of the alkali or alkaline earth metals, either alone or in further combination with oxides of boron, silicon, aluminum, phosphorus, magnesium, calcium or zirconium. The intended purpose of the glaze is to mechanically fill in the pores which occur in both the graphite substrate and in the silicon carbide or silicon oxicarbide coating. A problem inherent with this sort of a glaze is that, although it might be sufficient to prevent oxidation at elevated temperatures, it does not deal with the wear problem. Also, it adds the concern of having a coating on the exterior of the graphite article which may be reactive with high temperature fluids.
An especially effective technique has been discovered for forming a coating of silicon carbide on the surface of a graphite crucible in order to enable pure silicon to be melted in the crucible for contamination-free crystallization. Although it is believed that this technique has not been used in combination with a protective glaze to eliminate the cracking problem, it nevertheless should be effective in reducing problems associated with thermal shock in silicon carbide coatings. French Pat. No. 1,388,539 discloses a crucible composed of graphite wherein the inner surface of the crucible is transformed into silicon carbide by way of a multi-step process. The crucible first is elevated in temperature to 1300.degree. C. for a short period of time. Then hydrogen and trichlorosilicane gases are introduced into the crucible. This causes the formation of silicon on the surface of the graphite. Next, the temperature is elevated to 1420.degree. C. where the silicon begins to melt. Then the temperature is lowered to 1300.degree. C. and maintained there for a short period of time while the hydrogen and trichlorosilicane gases are continued to be circulated through the chamber. Again the temperature is raised to 1420.degree. C., causing the silicon to remelt. The temperature then is lowered again to 1300.degree. C. Then, again, the temperature is increased for a third time to 1420.degree. C. All of this causes a uniform melting of the silicon deposit on the graphite. Finally, the temperature of the crucible is raised slightly above 2000.degree. C., bringing the graphite up to the white heat stage, where it is held for about one minute. At this point, the silicon reacts with the graphite substrate to form a zone of silicon carbide. Since the crucible has been maintained continuously at relatively uniform elevated temperatures, there is reduced concern with cracking or flaking of the silicon carbide surface caused by thermal shock. Nevertheless, in spite of the relatively high quality silicon carbide resulting from the use of this technique, it still does not completely address the problem of oxidation of the graphite substrate. Also, the surface tolerances of the finished product render it largely unsuitable for use in environments employing closely mating parts.
One approach to the oxidation problem calls for the application of aluminum phosphate (AlPO.sub.4) compounds or magnesium phosphate (Mg.sub.3 (PO.sub.4).sub.2) compounds to graphite bodies to mechanically fill, or impregnate, the pores, thus reducing the propensity of the graphite to oxidize at elevated temperatures. For example, several formulations have been suggested for developing aluminum phosphate compounds which can act as impregnants; however, the molar ratio of aluminum oxide (Al.sub.2 O.sub.3) to phosphorus oxide (P.sub.2 O.sub.5) in the aluminum phosphate compounds must be between about 0.2:1 and 0.8:1. A distinction has been made between aluminum phosphate and aluminum phosphate dissolved in acidic solution because aluminum phosphate, as such, actually exerts a catalytic effect on the oxidation rate of graphite and other carbonaceous materials at temperatures in excess of 500.degree. C.
Yet another anti-oxidation technique carries on the concept of using aluminum phosphate compounds, as distinguished from pure aluminum phosphate, in coating graphite articles. This technique employs a compound called "calcium aluminum oxyphosphate." The compound is formed by the interaction of calcium, aluminum, and oxyphosphate-containing compounds which react to form a compound having calcium oxide (CaO), aluminum oxide (Al.sub.2 O.sub.3), and phosphorus oxide (P.sub.2 O.sub.5). The calcium aluminum oxyphosphate compound is brushed or sprayed onto the surface of the graphite article or the graphite article is soaked in the compound.
The foregoing techniques, which employ various acidic solutions of aluminum phosphate, as distinguished from only aluminum phosphate, all have served to increase to one degree or another the oxidation resistance of graphite articles at elevated temperatures. However, the increase has not been sufficient to significantly alter the economics involved with frequent replacements of these graphite articles. Importantly, none of the foregoing anti-oxidizing techniques has adequately dealt with the erosion problem caused by high temperature fluids coming into contact with the graphite articles.
A more recent approach is directed toward diminishing or eliminating the detrimental effects of oxidation of graphite articles, as well as to providing good erosion resistance. This more recent approach employs an aluminum coating chemically bonded to a graphite article by means of a metal carbide interface, the metal selected from the group consisting of tantalum, titanium and hafnium. The interfacial layer of metal carbide is synthesized in situ in the presence of aluminum, thereby effecting a strong bond between the aluminum and the carbon via the metal carbide interface. The advantage of this technique is that the aluminum is chemically bonded to the carbon, rather than mechanically. Accordingly, the graphite articles thus coated are useful in applications such as furnace electrodes where graphite articles having coatings of mechanically coated aluminum would not be suitable. Nevertheless, the principal uses of the technique are for bonding graphite articles together or for bonding a graphite article to another article. Even though the combination of aluminum and the metal carbide interfacial layer serves to enhance the oxidation resistance characteristics of the graphite article, the resistance to wear of the graphite article is not as great as desired.
In spite of the advances of prior art techniques as described previously for attempting to eliminate the problems of corrosion and wear in graphite articles, there still is a need for improvements in the corrosion and wear resistance characteristics of graphite articles, particularly when those articles are brought into contact with a flow of fluids at elevated temperatures in the presence of air.